Mouse Immunoglobulin Heavy Chain Gene Organization and Rearrangement: Genetic Bases for Antibody Diversity and Regulated Expression

Immunoglobulin heavy chains each display one of a wide range of diverse antigenbinding variable regions. At least one class of immunoglobulin, IgM, contains heavy chains which exist as two forms, either bound to the outside of a cell membrane or linked by disulfide bonds in secreted antibodies. I have used recombinant DNA techniques to isolate and determine the nucleotide sequences of genes encoding mouse immunoglobulin heavy chains. This has enabled me to examine genetic bases for the diversity of heavy chain variable regions and for the synthesis of membrane-bound and secreted forms of IgM heavy chains. I found that genes encoding heavy chain variable regions are created somatically by joining three segments of DNA: VH, D, and JH The JH gene segments are closely linked to the IgM heavy chain constant region gene in germline DNA, where they are widely separated from VH gene segments. In an immunoglobulin-producing cell, one VH and one JH gene segment are joined, together with a D sequence which is probably also a germline gene segment, to form the expressed heavy chain variable region gene. Both combinatorial association of gene segments and variations in the exact sites of DNA joining between gene segments can contribute to heavy chain variable region diversity. Based on observations of certain conserved nucleotides and spacer sequences adjacent to unrearranged immunoglobulin gene segments, I propose a mechanism for variable region gene rearrangement during differentiation. Secreted and membrane-bound forms of IgM heavy chains were found to be encoded by separate mRNAs transcribed from the same gene. These mRNAs differ only at their 3' ends, where one encodes a 20 amino acid secretory C-terminal segment, and the other encodes a 41 amino acid transmembrane C-terminal segment. Synthesis of the two forms of lgM heavy chain mRNA appears to be developmentally regulated by controlling the site of 3' terminal polyadenylation. The site of polyadenylation defines the lengths of RNA transcripts and thereby determines which of two alternative RNA splicing patterns will be followed, leading to mRNAs encoding either the secreted or membrane-bound forms of IgM heavy chains.